1. Technical Field
The present invention relates to a communication system in which data is transmitted and received among a plurality of communication apparatuses. More particularly, the present invention relates to relay apparatuses, a communication system including the relay apparatuses and a communication method, capable of shortening the time required for data exchange by reducing the amount of communication so as to reduce communication load in the case that data is exchanged among the relay apparatuses in a communication system provided with the plurality of relay apparatuses for relaying the transmission and reception of data.
2. Description of Related Art
In recent years, there has been used, in various fields, a system in which a plurality of communication apparatuses are connected, functions are assigned to the respective communication apparatuses so that data is exchanged mutually among the communication apparatuses and various processes are carried out in cooperation. For example, in the field of vehicle-mounted LAN (local area network) on vehicles, ECUs (Electronic Control Units) are used as communication apparatuses to carry out processes specific to the respective ECUs and to exchange data among them, whereby various functions are achieved in the system.
As the functions of the respective communication apparatuses are specialized and the functions that can be carried out by the respective communication apparatuses increase in number, the communication apparatuses to be connected to communication media also increase in number and type. Furthermore, since various functions are expected to be carried out in the system, it becomes necessary for the respective communication apparatuses to share data and operate in cooperation, whereby the amount of data to be transmitted increases.
The increase in the amount of data to be transmitted and received via a communication line may cause data delay or dropout due to collision. Significant data delay or dropout may be sometimes fatal to the driving assistance functions of a vehicle, such as brake control using an ECU.
Accordingly, there has been generally used a configuration which includes a plurality of communication lines and in which ECUs are connected to the respective communication lines. This configuration is adopted because wasteful use of communication lines can be suppressed by grouping ECUs in which data is shared. Furthermore, another configuration has also been used in which, for the purpose of using communication lines efficiently to cope with the increase in the type of ECUs, the ECUs are classified depending on the type of data to be transmitted and received and connected to communication lines, communication speeds of which are different. With these configurations, relay apparatuses for controlling data transmission and reception are connected among different communication lines.
Even in the case that the communication apparatuses are divided into a plurality of groups, the amount of data to be transmitted to the communication lines is not reduced in a configuration in which the relay apparatuses transfer all the data required for the control of the respective communication apparatuses. Hence, Japanese Patent Application Laid-Open No. 2007-300331 discloses a technology in which data received from each communication apparatus is stored once in a database, data required for each communication apparatus is read from the database and transmitted as necessary, and data recorded in the databases is transmitted and received among the apparatuses having databases, whereby the databases are shared.